The 15% reduced cooldown from Perfect Timing stacks multiplicitively with Active Item CDR when concerning Zhonya's Hourglass and Gargoyle Stoneplate. Guardian Angel is not considered an item-active, and so is only reduced by Perfect Timing and not reduced by any other source of cooldown reduction.
Passive: Receive a Replica Stopwatch at 6 minutes. The Replica Stopwatch is identical to the Stopwatch, except it has only half the value and so costs 300 more to upgrade. A cosmetic, Commencing Stopwatch, occupies one of your item slots before this time.